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J O U R N A L O F M A T E R I A L S S C I E N C E 2 3 ( 1 9 88 ) 2 5 0 0 - 2 5 0 4

A s t u d y o f t h e s t r u c t u r a l , e l e c t r i c a l a n d o p t i c a l

p r o p e r t ie s o f c o p p e r t e l lu r i u m o x i d e g la ss es

M . A . H A S S A N , C . A . H O G A R T H

Physics Department, Brun el University, Uxbr idge, M iddlesex, UK

The resu l t s o f an i nves t i ga t i on o f some proper t i es o f CuO-TeOz g lasses a re repor ted . X- ray

d i f f rac t i on , d i f fe r en t i a l s c ann i ng c a l o r im e t r y and op t i c a l ab s o r p t i on m eas u r em en t s a r e d i s -

c us s ed w i t h r es pec t t o t he c om pos i t ions . D .c . c o nd uc t i v i t y is m eas u r ed i n t he t em pe r a t u r e

r ange 293 t o 4 53 K and d i s c us s ed i n t e r m s o f s m a l l po l a r on t heo r y .

1.

I n t r o d u c t i o n

Several transition metal oxides when heated with

glass-forming substances such as PzOs, TeO2 and

GeO 2, etc., form glasses on quenching the melt. The

loss of oxygen from the melt produces a proportion of

lower valency transition metal ions, and electrical

conduction in these glasses occurs by electron hopp ing

from an ion in the low valency state (e.g. Cu +) tran-

sition metal to a n ion in the higher valency state

C u ~ + ) [ 1 1 .

Differential thermal analysis, X-ray diffraction and

electrical conductivity for CuO-TeO2 glasses have

been studied by Ivanova e t a l . [2]. They foun d that for

all the compositions examined, the dependence of

electrical conduct ivity, cr, as a function o f reciprocal

temperature (l/T) yields different activation energies

at low and high temperatures. Electrical conductivity

measurements for glasses of the ternary system CuO-

V2Os-TeO 2 have been investigated by Dimitriev e t a l .

[3]. They noted that the temperature dependence of

the conductance is better described by the Arrhenius

equation above 100~ and that the calculated acti-

vat ion energy varies within the limits 0.4 to 0.8 eV.

The physical parameters obtained from measure-

ments such as X-ray diffraction, differential scan-

ning calorimetry (DSC), and optical absorpt ion are

discussed with respect to the compositions. The con-

ductivity was measured for semiconducting copper

tellurium oxide glasses. The present studies show that

the conductivityof CuO-TeO2 glasses is slightly higher

than that of the corresponding CuO-P205 glasses [4].

2 . E x p e r i m e n t a l w o r k

Six glasses in the CuO-TeO 2 system with 15, 25, 30,

35, 40, and 50 mol % CuO were prepared by melting

the oxides in alumina crucibles for 30min a t 700 to

900 ~ C. The electrical measurements were made by

standard techniques. Evaporated gold electrodes were

found to give ohmic contacts for fields up to at least

103 V cm-~ and were used in al l the electrical measure-

ments. Thin glass films, which are necessary for optical

absorption measurements, were obtained by blowing

techniques. Specimens in the thickness range 2.5 to

6.25 m were obtained as measured using a Sigma

comparator. All thin films were unannealed when

used for optical'measurements. The optical absorp-

tion measurements were made on thin blown films at

room temperature in the wavelength range 200 to

900 nm.

3 . R e s u l t s a n d d i s c u s s i o n

3 . 1 . X - r ay d i f f r ac t i on

X-ray diffraction is a quite useful technique because it

is possible to detect readily crystals in a glassy matrix

if the crystals are of dimensions greater than typically

100 nm [5]. The X-ray diffraction pattern of an amorph-

ous material is distinctly different from that of crystal-

line materia l and consists of a few broad diffuse haloes

15 nlol % CuO

25 m01% Cu0

1 d i f f r a c t i on o f t h re e

igure

X - r a y

p a t t e r n s

sa mpl e s o f c op pe r - t e l l u r i um ox i de g l as ses .

2500 0 0 2 2 - 2 4 6 1 / 8 8 $ 0 3 .0 0 + . 12

9 1988 Chapman and Hall Ltd.

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T A B L E I E x o t h e r m a l a n d e n d o t h e r m a l t e m p e r a t u r e t r a n s it i o n

p e a k s f o r s o m e g l a s s s a m p l e s u s i n g D S C i n t h e ra n g e 5 0 t o 6 0 0 ~

C u O E n d o t h e r m a l E x o t h e r m a l - A H

( m o i % ) t e m p ( ~ t e m p ( ~ ( j g - l )

15 356 410 88 .26

436 472 45 .30

25 352 396 14.79

428 476 54 .24

T A B L E I I C h e m i c a l c o m p o s i t i o n an d d e r iv e d pa r a m e t e r s o f

C u O - T e O 2 g l as s es

Cu O W E e , ( = 2 W ) Eop~ E o

( m o l %) ( e V ) ( e V ) ( e V ) ( e V )

15 0.8 1.60 - -

25 0.74 1.48 2.75 0.33

30 0.69 1.38 2.68 0.34

35 0.66 1.32 2.62 0.37

40 0.60 1.20 2.55 0.49

50 0.53 1.06 2.30 0.75

r a t h e r t h a n s h a r p r i n g s . A l l g l a s s s a m p l e s w e r e t e s t e d

a n d t h e r e s u l t s s h o w e d t h e a b s e n c e o f c r y s t a l li n e

c h a r a c t e r i s t i c s . F i g . 1 s h o w s t y p i c a l X - r a y d i f f r a c t i o n

p a t t e r n s f o r t h r e e c o m p o s i t i o n s , r e c o r d e d w i t h a

P h i l l i p s t y p e P W 1 0 5 0 d i f f r a c t o m e t e r , u s i n g a c o p p e r

t u b e a n d n i c k e l f i l t e r .

3.2. Thermal analysis

I n t h i s i n v e s t i g a t i o n t h e d i f f e r e n t i a l s c a n n i n g c a l o r -

i m e t r y ( D S C ) t e c h n i q u e w a s u s e d . A n i m p o r t a n t u s e

o f D S C i n g l as s e s i s t o m e a s u r e t h e g l a s s t r a n s i t i o n

t e m p e r a t u r e , T g . A s a m p l e a n d a n i n e r t r e f e r e n c e a r e

u s e d a n d t h e c e l l i s d e s i g n e d t o e n a b l e a d i r e c t q u a n -

t i t a t i v e m e a s u r e m e n t f o r t h e e n t h a l p y c h a n g e s t h a t

o c c u r i n a s a m p l e a s a f u n c t i o n o f e it h e r t e m p e r a t u r e

o r t i m e t o b e m a d e .

T h e t h e r m a l s t a b i l i t y o f t h e g l a ss e s w a s s t u d i e d

i n a P e r k i n E l m e r D S C 7 e q u i p m e n t f o r t w o c o m p o -

s i t i o n s i n t h e t e m p e r a t u r e r a n g e 3 1 3 to 8 7 3 K . T h e

h e a t f l ow t o t h e s a m p l e w a s m e a s u r e d u n d e r t h e r m a l l y

c o n t r o l l e d c o n d i t i o n s . D S C g a v e m u c h m o r e s e n si ti v e

r e s u l t s s h o w i n g t h a t t h e t r a n s i t i o n s s t a r t e d a t a l o w e r

t e m p e r a t u r e t h a n e x p e c t e d . F i g . 2 s h o w s t y p i c a l c u r v e s

o f th e t r a n s i t i o n s a n d T a b l e I g i v e s a s u m m a r y f o r t h e

g l a ss e n d o t h e r m a l p e a k v a l u e s , e x o t h e r m a l p e a k v a l u e s

a n d t h e a m o u n t o f h e a t a b s o r b e d b y t h e g la s s i n t h e

e n d o t h e r m a l t r a n s i t i o n s .

3 .3 . Optica l absorpt ion

A m a i n f e a t u r e o f t h e a b s o r p t io n e d g e o f a m o r p h o u s

s e m i c o n d u c t o r s , p a r t i c u l a r l y a t t h e l o w e r v a l u e s o f

a b s o r p t i o n c o e f f ic i e n t , i s a n e x p o n e n t i a l i n c r e a s e o f

t h e a b s o r p t i o n c o e f fi c ie n t e ( c o) w i t h p h o t o n e n e r g y

(h ~o ) i n a c c o r d a n c e w i t h a n e m p i r i c a l r e l a t i o n d u e t o

Urbach [6 ]

c~ = ~o ex p

( h c o / E o )

(1)

wh ere c~0 i s a co ns ta n t , E0 i s the U rba ch ene rgy wh ich

i n d i c a t e s t h e w i d t h o f t h e b a n d t a i l s o f t h e l o c a l i z e d

s t a te s a n d m i s t h e a n g u l a r f r e q u e n c y o f t h e r a d i a t i o n .

O p t i c a l a b s o r p t i o n m e a s u r e m e n t s w e r e m a d e a s

a f u n c t i o n o f p h o t o n e n e r g y a t r o o m t e m p e r a t u r e .

F i g . 3 s h o w s t h e a b s o r b a n c e i n a r b i t r a r y u n i t s a s a

f u n c t i o n o f t h e w a v e l e n g t h f o r g l a s s fi l m s o f d i ff e r e n t

c o m p o s i t i o n s .

T h e a b s o r p t i o n c o e f f i c ie n t c~ (co) c a n b e d e t e r m i n e d

n e a r t h e e d g e f r o m t h e f o r m u l a

1 I 0

~ ( c ~ ) = ~

tn 7

( 2 )

w h e r e d i s t h e t h i c k n e s s o f t h e s a m p l e , I 0 a n d I

a r e t h e i n t e n s i t i e s o f t h e i n c i d e n t a n d t r a n s m i t t e d

b e a m s , r e s p e c t iv e l y . T h e m o s t s a t i s f a c t o r y re s u l t s w e r e

o b t a i n e d b y p l o t t i n g t h e q u a n t i t y ( ~ 0 ) ) 1/2 a s a func-

t i o n o f ( hc o) a s s u g g e s t e d b y D a v i s a n d M o t t [ 7] . F o r

a b s o r p t i o n b y i n d i r e c t tr a n s i t io n s t h e e q u a t i o n t a k e s

t h e f o r m

~ ( c o ) = A ( h c o - E o p t ) 2 / h c o

(3)

w h e r e A i s a c o n s t a n t , E o p t i s t h e b a n d g a p a n d t h i s

40 -

30-

E

g

2 0

10-

~

.d

I I I

100 300

Tempe rature ( o C)

s ~ o

Figure 2 T y p i c a l D S C c u r v e s f o r t w o

c o p p e r - t e l l u r i u m o x i d e g l a s s e s . ( - - - )

2 5 m o I % Cu O , ( ) 1 5 t o o l % Cu O .

2501

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3/5

/

" / ~ . . _ I 9

=0.5, / .

- ]

~ i /

.~

o

. a ]

1 . 0 / ,

/ , ' /

I .

, /

I.

1.5.

4 0 0 5 0 0 6 0 0 7 0 0

Wavelength (nrn)

Figure 3 Absorban ce as a function of wav elength for three samp les

of copper-tel lurium oxide glasses. ( ) 25 mol % C uO , ( - - - )

35 mol % CuO , ( - . . . . . . ) 50 tool % CuO.

r e l a t i o n a p p l i e s t o m a n y o x i d e g l a ss e s , p a r t i c u l a r l y a t

t h e h i g h e r v a l u e s o f a b s o r p t i o n c o e ff ic i en t . F i g s 4 a n d

5 s h o w p l o t s o f ( e hc o) 1/2 a g a i n s t h e ) a n d t h e e q u i v a l e n t

U r b a c h p l o t i s p r e s e n t e d i n F i g s 6 a n d 7 in w h i c h t h e

a b s o r p t i o n c o e f f i c ie n t s a r e p l o t t e d a s f u n c t i o n s o f hc o

180

1 6 0

T

5140

3

120

100

I n l

(e)

/ I t I I

2 . 0 2 .5 3 . 0 3 . 5

r

l

/

/

/ a / ~ J

/ 1 o i

/ ~ / i

Photon energy eV)

F i g u r e 4 (~hco) /2 plotted against hco for three samples o f co ppe r-

te l lu r ium oxide g lasses. (a ) 50mo1% CuO, 2 .5#m; (b) 35m o1%

CuO, 2 .7#m; (c ) 25mo1% CuO, 2 .6#m.

(o) (b)

100 -

r a

>o

' E 8 0

6 O

/

/ /

/ /

I

I I I I

2.0 2.5 3 . 0 3,5

Pho ton e nergy (eV)

Figure 5 c~hc@/2

p lo t ted aga ins t he ) fo r two samp les o f copper -

te l lu r ium ox ide g lassesof the same th ickness, 6 .25 f i re. (a) 40 tool %

CuO, (b) 30 tool % CuO.

f o r d i f fe r en t c o m p o s i t io n s o f C u O - T e O 2 g l as se s . T h e

v a l u e s o f E ov d e t e r m i n e d f r o m F i g s 4 a n d 5 b y e x t r a -

p o l a t i n g t h e l in e a r p a r t s o f th e c u r v e s to (0~(D ) 1/2 = 0,

a n d t h e v a l u e s o f E 0 w h i c h a r e d e t e r m i n e d f r o m

F i g s 6 a n d 7 b y u s i n g E q u a t i o n 1 a r e t a b u l a t e d i n

T a b l e I I . I t is c l e a r f r o m F i g . 8 t h a t t h e o p t i c a l g a p E o p ,

d e c r e a s e s a s th e p r o p o r t i o n o f tr a n s i t i o n m e t a l o x i d e

C u O i s i n c r e a s e d , b u t o n t h e o t h e r h a n d t h e w i d t h o f

t h e b a n d t a i ls o f th e l o c a l i z e d s t a t e s is f o u n d t o

i n c r e a s e w i t h i n c r e a s i n g C u O c o n t e n t , a s s h o w n i n

F i g . 9 . T h e v a l u e o f E op t i s c o n s i d e r a b l y l a r g e r t h a n

t w i c e t h e e l e c t r i c a l a c t i v a t i o n e n e r g y , W .

3 . 4 . D . c .

c o n d u c t i v i t y

T h e d . c . e l e ct r ic a l c o n d u c t i v i t y o f C u O - T e O 2 g l a s s e s

w a s m e a s u r e d i n t h e t e m p e r a t u r e r a n g e 2 9 3 t o 45 3 K .

A l l m e a s u r e m e n t s w e r e t a k e n m o r e t h a n o n c e . T h e

9.0

'E

v

8.0

(a).

~

c)

I I I

2.0 2.5 3.0 3.5

Pho ton energy (eV)

Figure 6 Ln ~ plotted against he) for the three sa mples of Fig. 4.

2 5 0 2

8/18/2019 Hassan 1988

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8~-

JE

2.0

( b )

I I I

2.5 3.0 3.5

Photon energy (eV)

Figure 7 L n e p l o t t e d a ga i ns t he ) fo r t he t w o sa mpl e s o f F i g . 5 .

d . c . c o n d u c t i v i t y , aa .. . i n c r e a s e s w i t h t e m p e r a t u r e a n d

a l s o w i t h t h e i n c re a s e in t h e c o n c e n t r a t i o n o f t r a n s i t i o n

m e t a l o x i d e C u O i n t h e g l as s , a s s h o w n i n F i g . 1 0 . A l l

g l as se s s h o w a s m o o t h v a r i a t i o n o f c o n d u c t i v i t y w i th

( 1 0 0 0 / T ) K - ~ . T h e d . c. c o n d u c t i v i t y i s a n e g a t i v e

e x p o n e n t i a l f u n c t i o n o f i n v er s e t e m p e r a t u r e a n d c a n

b e e x p r e s s e d b y t h e u s u a l e q u a t i o n

O-d.c. = a0 e x p - (4 )

w h e r e W i s t h e a c t i v a t i o n e n e r g y .

T h e a c t i v a t i o n e n e r g i e s w e r e c a l c u l a t e d f r o m t h e

s lopes o f the log ~d .c . aga in s t

I/T

c u r v e s a n d w e r e

f o u n d t o d e c r e a s e a s th e c o n c e n t r a t i o n o f t ra n s i t i o n

m e t a l o x i d e C u O i n c r e a s e s i n t h e g l a s s , a s s h o w n i n

F i g . I I . T h e v a l u e s o f t h e a c t i v a t i o n e n e r g y , W , a b o v e

r o o m t e m p e r a t u r e a r e t y p i c a l l y 0 .6 1 t o 0 .8 e V a n d a r e

2.8

2.6

~o

2.4

2.2

25 35 45

Cu0 con ten t ( mot % }

Figure 8 Fop

p l o t t e d a g a i n s t C u O c o n t e n t f o r c o p p e r - t e l l u r i u m

oxide glasses.

0.~

0.6

>=

0.4

0.2

I I

25 35 45

CuO content {tool o/o)

Figure 9 Eo p l o t t e d a g a i n s t C u O c o n t e n t f o r c o p p e r - t e l l u r i u m o x i d e

glasses.

c o n s i d e r a b l y l e s s t h a n h a l f t h e c a l c u l a t e d v a l u e s o f t h e

o p t i c a l g a p , a s s h o w n i n T a b l e I I .

T h e d . c . e l e c tr i c a l c o n d u c t i v i t y a n d t h e s h a p e o f th e

c u r r e n t - v o l t a g e c h a r a c t e r i st i c s w e r e s e n s it i ve t o t e m -

p e r a t u r e . T h e g l a s s e s s h o w o h m i c b e h a v i o u r a t l o w

f ie lds , a s s how n in F ig . 12 . As can b e s een f rom

F i g . 1 3 , th e r e s i s t a n c e o f C u O - T e O 2 g l a s s es i s i n d e -

p e n d e n t o f ti m e . T h i s c o u l d b e t a k e n a s e v id e n c e t h a t

t h e p o l a r i z a t i o n e f f e c t i n t h e s e g l a ss e s is m i n i m a l o r

a b s e n t a n d t h e e l e c t r ic a l c o n d u c t i o n w o u l d i n t h i s c a s e

b e d u e t o t r a n s p o r t o f e l e c tr o n s r a t h e r t h a n i o n s.

I 0 -6 -

•E

0-7

T

10 a

8

10-9

10-10 I ~ t

2.0 2.5 3.0 3.5

1000 /T (K -1 )

Figure 10 C o n d u c t i v i t y a s a f u n c t i o n o f i n v e r se t e m p e r a t u r e f o r f o u r

s a m p l e s o f c o p p e r - t e l l u r i u m o x i d e g la ss . ( a ) 1 5 m o 1 % C u O ,

(b ) 25 t oo l % C uO , ( c ) 30 t oo l % C uO , (d ) 40 t oo l % C uO.

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1.0

05

I 1 I i

15 25 35 ~5

CuO content (rno[ O/o)

Figure l l V a r i a t i o n o f a c t i v a ti o n e n e r g y , W , w it h C u O c o n t e n t o f

c o p p e r - t e l l u r i u m o x i d e g l a s s .

4 . D i s c u s s i o n

I t i s o f i n t e re s t t o d i s c u s s t h e f o l l o w i n g p o i n t s .

( i) T h e s h i f t o f t h e u l t r a v i o l e t a b s o r p t i o n b a n d t o

l o n g e r w a v e l e n g t h w i t h t h e i n c r e a s i n g C u O c o n t e n t .

( ii ) T h e v a r i a t i o n o f E o pt w i t h e l e c t ri c a l a c t i v a t i o n

e n e r g y .

(i ii ) T h e k i n d o f c o n d u c t i o n m e c h a n i s m w h i c h i s

l i k e l y to b e i n v o l v e d i n t h e s e m a t e r i a l s .

( i ) S t e v e l s [ 8 ] s u g g e s t s t h a t t h e m o v e m e n t o f t h e

u l t r a v i o l e t a b s o r p t i o n b a n d t o l o n g e r w a v e l e n g t h s

c o r r e s p o n d s t o a t r a n s i t i o n f r o m t h e n o n - b r i d g i n g

o x y g e n s w h i c h b i n d a n e x c i t e d e l e c t r o n l e s s t i g h t l y

t h a n a b r i d g i n g o x y g e n . A s c a n b e s e e n f r o m F i g . 3 th e

s h i f t o f t h e u l t r a v i o l e t a b s o r p t i o n b a n d t o l o n g e r

w a v e l e n g t h s w i t h i n c r e a s i n g C u O c o n t e n t c o u l d b e

e x p l a i n e d i n t h e s a m e w a y . T h u s t h e r e s u l t o f F i g . 8

s h o w i n g t h e v a r i a t i o n o f E op~ w i t h c o m p o s i t i o n c a n b e

e x p l a i n e d b y s u g g e s t in g t h a t t h e n o n - b r i d g i n g o x y g e n

(d)

_ _ c )

(b)

10-9 (a)

10-10

10- 1

10-12 I

10-13 [ i I

1 o 1o o 1o o o

Applied voltoge V )

Figure 12 V I c h a r a c t e ri s t ic s a t r o o m t e m p e r a t u r e o f t h e f o u r

s a m p l e s o f F i g . 1 0.

2 5 0 4

1 0 1 1

1 0 1 0

c

c ~

1 0 9

- - 2 0 C

5 0 ~ C

S

z ~- -- e ~ - e _- e

80 ~ C

1 2 0 ~

1 0 8 I l

I

1 2 3

T im e h )

Figure 13

Variation of resistance with tim e for a 30mo1% CuO

sample at different emperatures.

i o n c o n t e n t i n c r e a s e s w i t h i n c r e a s i n g C u O c o n t e n t ,

s h i f t i n g t h e b a n d e d g e t o l o w e r e n e r g i e s a n d t h u s

c o n s e q u e n t l y l e a d i n g t o a d e c r e a s e i n E o p t .

( ii ) As c an be seen f i ' om Tab le I I , th e so -ca l led

o p t i c a l g a p ( Eo pt ) i s t h r e e t o f o u r t i m e s l a r g e r t h a n t h e

h i g h - t e m p e r a t u r e a c t i v a t i o n e n e r g y . T h i s i s c o n s i s t e n t

w i t h t h e r e s u lt s o f m a n y w o r k e r s o n o x i d e a n d c h a l-

c o g e n i d e g l a s se s w h i c h s h o w t h a t t h e e l e c t r i c a l e n e r g y

g a p , s o m e t i m e s r e g a r d e d a s t w i c e t h e a c t i v a t i o n e n e r g y ,

W , is le s s t h a n t h e v a l u e o f th e o p t i c a l g a p o n t h e s a m e

g l a ss . T h e r e s u l t s s u g g e s t t h a t t h e e l e c t r o n i c a c t i v a t i o n

i s n o t a c r o s s t h e w h o l e m o b i l i t y g a p b u t i s p o s s i b l y

f r o m o n e o r m o r e t r a p p i n g l e v e l s t o t h e c o n d u c t i o n

b a n d o r f r o m b o n d i n g s t a t e s t o a t r a p p i n g l e v e l .

( ii i) T h e l o g o- a g a i n s t r e c i p r o c a l t e m p e r a t u r e p l o t

d i s p l a y s l in e a r b e h a v i o u r w h i c h i s t y p i c a l o f t h e s m a l l

p o l a r o n h o p p i n g m e c h a n i s m e x h i b i t e d b y 3 d a n d 4 d

t r a n s i t i o n m e t a l o x i d e g l a s s e s .

R e f e r e n c e s

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R e c e i v e d 7 J u l y

a n d a c c e p t e d 9 O c t o b e r 1 9 8 7